1. Field of the Invention
The present invention relates to an ink jet recording apparatus which jets ink in droplets form onto a recording medium to form an image thereon.
2. Description of the Related Art
Recently, in an ink jet image forming method, an image can be formed easily and at low cost in comparison with a gravure printing method, so that the ink jet image forming method has been applied in various printing fields such as special printing and the like, that is, a photograph, various printings, marking, color filter and so forth.
Particularly, in the ink jet image forming method, an ink jet recording apparatus of the ink jet image forming method which jets and controls a small dot, ink which has an improved gamut of color reproduction, endurance, aptitude for jetting ink and the like, and an specific paper which has dramatically improved ink absorptivity, coloring property of coloring material, surface gloss and the like are combined for use so as to obtain image quality comparable to a silver salt photograph.
An improved image quality of the ink jet image forming method can be obtained when the ink jet recording apparatus, the ink and the specific paper are all provided. However, if the recording apparatus needs the specific paper, the type of recording medium is limited. This results in high cost for recording medium and restricted application of the ink jet recording apparatus. Thus, lots of attempts have been made to form an image by the ink jet image forming method on a recording medium other than the specific paper. To put it more concretely, that is a phase-change ink jet method using wax ink which is solid at room temperature, a solvent-based ink jet method using an ink which main component is a quick-drying organic solvent, a UV ink jet method in which ink is irradiated with ultraviolet (UV) after forming an image and the like.
Particularly, in the UV ink jet method, less odor is produced in comparison with the solvent-based ink jet method. Moreover, an image can be formed on a recording medium which is no quick-drying property and absorptivity. Thus, recently, much attention has been paid to the UV ink jet method. For example, Japanese Patent Application Publication (Examined) No. Tokukou-Hei 5-54667, Japanese Patent Application (Unexamined) No. Tokukai-Hei 6-200204 and Patent Application Kohyo Publication (National Publication of Japanese Translation Version) No. Tokuhyo 2000-504778 disclose an ultraviolet curable ink (hereinafter referred to simply as UV ink).
However, the diameter of a dot changes a lot after placing the ink according to the working environment or the type of a recording medium even if the UV ink is used. Thus, there is less possibility to form an image with a high quality and a high-definition on any type of recording medium.
In a usual UV ink, there is a problem that the UV ink shrinks while curing, so that a recording medium shrinks as the UV ink shrinks. Specially, the ink shrinks more while curing on a thin plastic film, an adhesive label or the like which is used for soft packing including food packing. Accordingly, the UV ink jet method has not been put to practical use in printing on the soft packing or label printing.
Radical polymerization ink and cationic polymerization ink have been known as the UV ink. The inventors have found that shrinkage of the ink which occurs while forming an image can be prevented by using the cationic polymerization ink, specially the cationic polymerization ink with a specific composition, even if the jetted ink is received on the above-described thin plastic film or adhesive label.
However, the curing property of the cationic polymerization ink is easily affected by water (humidity). For example, when the cationic polymerization ink is received on the recording medium to form an image, there is a problem that the cationic polymerization ink is not cured enough due to an effect of ambient humidity even if UV radiation is irradiated on the cationic polymerization ink.
A UV ink jet recording apparatus jets the UV ink from a head toward the recording medium. When the UV ink which is jetted from the UV ink jet recording apparatus is received on the recording medium, the UV ink with droplets form spreads on a surface of the recording medium to form a UV ink dot on the recording medium. It is preferable that each diameter of a plurality of dots which are formed on the recording medium does not differ a lot for forming a high-definition image on the recording medium. However, if the cationic polymerization ink is not cured in a short period of time after irradiation of UV radiation due to an effect of ambient humidity as described above, each diameter of the dots greatly differs. Accordingly, in light of forming a high-definition image, the effect of humidity cannot be neglected.